Control method for process of removing permanganate reducing compounds from methanol carbonylation process

ABSTRACT

Disclosed is a method of controlling a separation process for removing permanganate reducing compounds from a process stream in the methanol carbonylation process for making acetic acid, where the method includes the steps of measuring the density of a stream containing acetaldehyde and methyl iodide, optionally calculating the relative concentrations of acetaldehyde and methyl iodide in the stream, and adjusting distillation or extraction process parameters based on the measured density or one or more relative concentrations calculated therefrom.

This application claims priority as a divisional application of U.S. patent application 10/708,422, filed Mar. 2, 2004, now U.S. Pat. No. 7,271,293 which is incorporated by reference herein in its entirety.

BACKGROUND OF INVENTION

1. Field of the Invention

This invention relates to an improved process for the removal of permanganate reducing compounds and alkyl iodides formed by the carbonylation of methanol in the presence of a Group VIII metal carbonylation catalyst. More specifically, this invention relates to an improved process for reducing and/or removing precursors of permanganate reducing compounds and alkyl iodides from intermediate streams during the formation of acetic acid by said carbonylation processes.

2. Technical Background

Among currently employed processes for synthesizing acetic acid, one of the most useful commercially is the catalyzed carbonylation of methanol with carbon monoxide as taught in U.S. Pat. No. 3,769,329 issued to Paulik et al. on Oct. 30, 1973. The carbonylation catalyst comprises rhodium, either dissolved or otherwise dispersed in a liquid reaction medium or supported on an inert solid, along with a halogen-containing catalyst promoter as exemplified by methyl iodide. The rhodium can be introduced into the reaction system in any of many forms, and the exact nature of the rhodium moiety within the active catalyst complex is uncertain. Likewise, the nature of the halide promoter is not critical. The patentees disclose a very large number of suitable promoters, most of which are organic iodides. Most typically and usefully, the reaction is conducted by continuously bubbling carbon monoxide gas through a liquid reaction medium in which the catalyst is dissolved.

An improvement in the prior art process for the carbonylation of an alcohol to produce the carboxylic acid having one carbon atom more than the alcohol in the presence of a rhodium catalyst is disclosed in commonly assigned U.S. Pat. Nos. 5,001,259, issued Mar. 19, 1991; 5,026,908, issued Jun. 25, 1991; and 5,144,068, issued Sep. 1, 1992; and European Patent No. EP 0 161 874 B2, published Jul. 1, 1992. As disclosed therein, acetic acid is produced from methanol in a reaction medium containing methyl acetate, methyl halide, especially methyl iodide, and rhodium present in a catalytically effective concentration. These patents disclose that catalyst stability and the productivity of the carbonylation reactor can be maintained at surprisingly high levels, even at very low water concentrations, i.e. 4 weight percent or less, in the reaction medium (despite the general industrial practice of maintaining approximately 14-15 wt % water) by maintaining in the reaction medium, along with a catalytically effective amount of rhodium and at least a finite concentration of water, a specified concentration of iodide ions over and above the iodide content which is present as methyl iodide or other organic iodide. The iodide ion is present as a simple salt, with lithium iodide being preferred. The patents teach that the concentration of methyl acetate and iodide salts are significant parameters in affecting the rate of carbonylation of methanol to produce acetic acid, especially at low reactor water concentrations. By using relatively high concentrations of the methyl acetate and iodide salt, one obtains a surprising degree of catalyst stability and reactor productivity even when the liquid reaction medium contains water in concentrations as low as about 0.1 wt %, so low that it can broadly be defined simply as “a finite concentration” of water. Furthermore, the reaction medium employed improves the stability of the rhodium catalyst, i.e. resistance to catalyst precipitation, especially during the product recovery steps of the process. In these steps, distillation for the purpose of recovering the acetic acid product tends to remove from the catalyst the carbon monoxide which in the environment maintained in the reaction vessel, is a ligand with stabilizing effect on the rhodium. U.S. Pat. Nos. 5,001,259, 5,026,908 and 5,144,068 are herein incorporated by reference.

It has been found that although a low water carbonylation process for producing acetic acid reduces such by-products as carbon dioxide, hydrogen, and propionic acid, the amount of other impurities, present generally in trace amounts, is also increased, and the quality of acetic acid sometimes suffers when attempts are made to increase the production rate by improving catalysts, or modifying reaction conditions.

These trace impurities affect quality of acetic acid, especially when they are recirculated through the reaction process. The impurities that decrease the permanganate time of the acetic acid include carbonyl compounds and unsaturated carbonyl compounds. As used herein, the phrase “carbonyl” is intended to mean compounds that contain aldehyde or ketone functional groups, which compounds may or may not possess unsaturation. See Catalysis of Organic Reaction 75, 369-380 (1998), for further discussion on impurities in a carbonylation process.

The present invention is directed to reducing and/or removing permanganate reducing compounds (PRC's) such as acetaldehyde, acetone, methyl ethyl ketone, butyraldehyde, crotonaldehyde, 2-ethyl crotonaldehyde, and 2-ethyl butyraldehyde and the like, and the aldol condensation products thereof. The present invention also leads to reduction of propionic acid.

The carbonyl impurities described above, such as acetaldehyde, may react with iodide catalyst promoters to form multi-carbon alkyl iodides, e.g., ethyl iodide, butyl iodide, hexyl iodide and the like. It is desirable to remove alkyl iodides from the reaction product because even small amounts of these impurities in the acetic acid product tend to poison the catalyst used in the production of vinyl acetate, the product most commonly produced from acetic acid. The present invention is thus also directed to removal of alkyl iodides, in particular C₂₋₁₂ alkyl iodide compounds. Accordingly, because many impurities originate with acetaldehyde, it is a primary objective to remove or reduce the acetaldehyde and alkyl iodide content in the process.

Conventional techniques to remove impurities include treating the acetic acid product with oxidizers, ozone, water, methanol, activated-carbon, amines, and the like, which treatment may or may not be combined with distillation of the acetic acid. The most typical purification treatment involves a series of distillations of the final product. It is also known, for example from U.S. Pat. No. 5,783,731, to remove carbonyl impurities from organic streams by treating the organic streams with an amine compound such as hydroxylamine, which reacts with the carbonyl compounds to form oximes, followed by distillation to separate the purified organic product from the oxime reaction products. However, the additional treatment of the final product adds cost to the process, and distillation of the treated acetic acid product can result in additional impurities being formed.

While it is possible to obtain acetic acid of relatively high purity, the acetic acid product formed by the low-water carbonylation process and purification treatment described above frequently remains somewhat deficient with respect to the permanganate time due to the presence of small proportions of residual impurities. Since a sufficient permanganate time is an important commercial test, which the acid product must meet to be suitable for many uses, the presence of impurities that decrease permanganate time is objectionable. Moreover, it is not economically or commercially feasible to remove minute quantities of these impurities from the acetic acid by distillation because some of the impurities have boiling points close to that of the acetic acid product.

It has thus become important to identify economically viable methods of removing impurities elsewhere in the carbonylation process without contaminating the final product or adding unnecessary costs. U.S. Pat. No. 5,756,836, incorporated herein by reference, discloses a method for manufacturing high purity acetic acid by adjusting the acetaldehyde concentration of the reaction solution below 1500 ppm. It is stated that by maintaining the acetaldehyde concentration below this threshold, it is possible to suppress the formation of impurities such that one need only distill the crude acetic acid product to obtain high purity acetic acid.

European Patent No. EP 0 487 284 B1, published Apr. 12, 1995, discloses that carbonyl impurities present in the acetic acid product generally concentrate in the overhead from the light ends column. Accordingly, the light ends column overhead is treated with an amine compound (such as hydroxylamine), which reacts with the carbonyl compounds to form oxime derivatives that can be separated from the remaining overhead by distillation, resulting in an acetic acid product with improved permanganate time.

European Patent Application No. EP 0 687 662 A2 and U.S. Pat. No. 5,625,095 describe a process for producing high purity acetic acid in which an acetaldehyde concentration of 400 ppm or less is maintained in the reactor by using a single or multi-stage distillation process to remove acetaldehyde. Streams suggested for processing to remove acetaldehyde include a light phase containing primarily water, acetic acid and methyl acetate; a heavy phase containing primarily methyl iodide, methyl acetate and acetic acid; an overhead stream containing primarily methyl iodide and methyl acetate; or a recirculating stream formed by combining the light and heavy phase. These references do not identify which of these streams possesses the greatest concentration of acetaldehyde.

EP 0 687 662 A2 and U.S. Pat. No. 5,625,095 also disclose management of reaction conditions to control the formation of acetaldehyde in the reactor. Although it is stated that formation of by-products such as crotonaldehyde, 2-ethylcrotonaldehyde, and alkyl iodides is reduced by controlling the formation of acetaldehyde, it is also pointed out that management of reaction conditions as proposed increases the formation of propionic acid, an undesirable by-product.

More recently, it has been disclosed in commonly assigned U.S. Pat. Nos. 6,143,930 and 6,339,171 that it is possible to significantly reduce the undesirable impurities in the acetic acid product by performing a multi-stage purification on the light ends column overhead. These patents disclose a purification process in which the light ends overhead is distilled twice, in each case taking the acetaldehyde overhead and returning a methyl iodide rich residuum to the reactor. The acetaldehyde-rich distillate is extracted with water to remove the majority of the acetaldehyde for disposal, leaving a significantly lower acetaldehyde concentration in the raffinate that is recycled to the reactor. U.S. Pat. Nos. 6,143,930 and 6,339,171 are incorporated herein by reference in their entirety.

While the above-described processes have been successful in removing carbonyl impurities from the carbonylation system and for the most part controlling acetaldehyde levels and permanganate time problems in the final acetic acid product, further improvements can still be made. Accordingly, there remains a need for alternative solutions to this problem to improve the efficiency and cost effectiveness of acetaldehyde removal. The present invention provides one such alternative solution.

SUMMARY OF INVENTION

In one aspect, the present invention provides a process for separating acetaldehyde from methyl iodide by distillation. The method includes the steps of distilling a mixture containing methyl iodide and acetaldehyde in a distillation apparatus to produce an overhead and a residuum; measuring the density of the overhead; and adjusting at least one process variable associated with said distillation apparatus in response to the measured density or a relative concentration calculated therefrom, where the process variable is selected from heating rate, column pressure, feed composition, reflux composition and reflux ratio.

In another aspect, the present invention provides a process for separating acetaldehyde from methyl iodide. The method includes the steps of distilling a mixture containing methyl iodide and acetaldehyde in a distillation apparatus to produce an overhead and a residuum; extracting the overhead with water to provide an aqueous extract and a raffinate; measuring the density of at least one of the overhead, the extract and the raffinate; and adjusting at least one process variable associated with the distillation apparatus or the extraction step in response to the measured density or a relative concentration calculated therefrom. The adjusted process variable is selected from the group consisting of column heating rate, column pressure, feed composition, reflux composition, and reflux ratio in the distillation apparatus, and water feed rate to the extractor.

In still another aspect, the present invention provides a process for producing acetic acid. The process includes the following steps: reacting methanol with carbon monoxide in a reaction medium containing water and methyl iodide in the presence of a catalyst; separating the reaction medium into a vapor phase containing acetic acid, methyl iodide, acetaldehyde and water and a liquid phase; distilling the vapor phase to produce a purified acetic acid product and a first overhead containing methyl iodide and acetaldehyde; condensing the first overhead and extracting it with water to produce an aqueous extract and a raffinate containing methyl iodide; measuring the density of at least one of the first overhead, the aqueous extract and the raffinate; and adjusting at least one process control parameter associated with the distillation or extraction of the second overhead based on the measured density or on a relative concentration of acetaldehyde or methyl iodide calculated therefrom.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates the prior art process, as disclosed in U.S. Pat. No. 6,339,171, for the removal of carbonyl impurities from an intermediate stream of the carbonylation process for the production of acetic acid by a carbonylation reaction.

FIGS. 2-4 illustrate preferred embodiments of the present invention in which a densitometer is located on a stream having the same composition as the overhead from the second distillation column.

FIG. 5 illustrates correlations between the measured density of overhead stream 54 and the respective concentrations of acetaldehyde and methyl iodide in that stream.

While the invention is susceptible to various modifications and alternative forms, specific embodiments have been shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention is intended to cover all modifications, equivalents and alternatives falling within the scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.

The purification process of the present invention is useful in any process used to carbonylate methanol to acetic acid in the presence of a Group VIII metal catalyst such as rhodium and an iodide promoter. A particularly useful process is the low water rhodium-catalyzed carbonylation of methanol to acetic acid as exemplified in U.S. Pat. No. 5,001,259. Generally, the rhodium component of the catalyst system is believed to be present in the form of a coordination compound of rhodium with a halogen component providing at least one of the ligands of such coordination compound. In addition to the coordination of rhodium and halogen, it is also believed that carbon monoxide coordinates with rhodium. The rhodium component of the catalyst system may be provided by introducing into the reaction zone rhodium in the form of rhodium metal, rhodium salts such as the oxides, acetates, iodides, etc., or other coordination compounds of rhodium, and the like.

The halogen-promoting component of the catalyst system consists of a halogen compound comprising an organic halide. Thus, alkyl, aryl, and substituted alkyl or aryl halides can be used. Preferably, the halide promoter is present in the form of an alkyl halide in which the alkyl radical corresponds to the alkyl radical of the feed alcohol, which is carbonylated. Thus, in the carbonylation of methanol to acetic acid, the halide promoter will comprise methyl halide, and more preferably methyl iodide.

The liquid reaction medium employed may include any solvent compatible with the catalyst system and may include pure alcohols, or mixtures of the alcohol feedstock and/or the desired carboxylic acid and/or esters of these two compounds. The preferred solvent and liquid reaction medium for the low water carbonylation process comprises the carboxylic acid product. Thus, in the carbonylation of methanol to acetic acid, the preferred solvent is acetic acid.

Water is contained in the reaction medium but at concentrations well below that which has heretofore been thought practical for achieving sufficient reaction rates. It has previously been taught that in rhodium-catalyzed carbonylation reactions of the type set forth in this invention, the addition of water exerts a beneficial effect upon the reaction rate (U.S. Pat. No. 3,769,329). Thus most commercial operations run at water concentrations of at least about 14 wt %. Accordingly, it is quite unexpected that reaction rates substantially equal to and above reaction rates obtained with such high levels of water concentration can be achieved with water concentrations below 14 wt % and as low as about 0.1 wt %.

In accordance with the carbonylation process most useful to manufacture acetic acid according to the present invention, the desired reaction rates are obtained even at low water concentrations by including in the reaction medium methyl acetate and an additional iodide ion which is over and above the iodide which is present as a catalyst promoter such as methyl iodide or other organic iodide. The additional iodide promoter is an iodide salt, with lithium iodide being preferred. It has been found that under low water concentrations, methyl acetate and lithium iodide act as rate promoters only when relatively high concentrations of each of these components are present and that the promotion is higher when both of these components are present simultaneously (U.S. Pat. No. 5,001,259). The concentration of lithium iodide used in the reaction medium of the preferred carbonylation reaction system is believed to be quite high as compared with what little prior art there is dealing with the use of halide salts in reaction systems of this sort. The absolute concentration of iodide ion content is not a limitation on the usefulness of the present invention.

The carbonylation reaction of methanol to acetic acid product may be carried out by contacting the methanol feed, which is in the liquid phase, with gaseous carbon monoxide bubbled through a liquid acetic acid solvent reaction medium containing the rhodium catalyst, methyl iodide promoter, methyl acetate, and additional soluble iodide salt, at conditions of temperature and pressure suitable to form the carbonylation product. It will be generally recognized that it is the concentration of iodide ion in the catalyst system that is important and not the cation associated with the iodide, and that at a given molar concentration of iodide the nature of the cation is not as significant as the effect of the iodide concentration. Any metal iodide salt, or any iodide salt of any organic cation, or quaternary cation such as a quaternary amine or phosphine or inorganic cation can be used provided that the salt is sufficiently soluble in the reaction medium to provide the desired level of the iodide. When the iodide is added as a metal salt, preferably it is an iodide salt of a member of the group consisting of the metals of Group IA and Group IIA of the periodic table as set forth in the “Handbook of Chemistry and Physics” published by CRC Press, Cleveland, Ohio, 1975-76 (56th edition). In particular, alkali metal iodides are useful, with lithium iodide being preferred. In the low water carbonylation process most useful in this invention, the additional iodide over and above the organic iodide promoter is present in the catalyst solution in amounts of from about 2 to about 20 wt %, the methyl acetate is present in amounts of from about 0.5 to about 30 wt %, and the lithium iodide is present in amounts of from about 5 to about 20 wt %. The rhodium catalyst is present in amounts of from about 200 to about 2000 parts per million (ppm).

Typical reaction temperatures for carbonylation will be approximately 150 to about 250° C., with the temperature range of about 180 to about 220° C. being the preferred range. The carbon monoxide partial pressure in the reactor can vary widely but is typically about 2 to about 30 atmospheres, and preferably, about 3 to about 10 atmospheres. Because of the partial pressure of by-products and the vapor pressure of the contained liquids, the total reactor pressure will range from about 15 to about 40 atmospheres.

A typical reaction and acetic acid recovery system that is used for the iodide-promoted rhodium catalyzed carbonylation of methanol to acetic acid is shown in FIG. 1 and comprises a carbonylation reactor, flasher, and a methyl iodide acetic acid light ends column 14 which has an acetic acid side stream 17 which proceeds to further purification. The reactor and flasher are not shown in FIG. 1. These are considered standard equipment now well known in the carbonylation process art. The carbonylation reactor is typically either a stirred vessel or bubble column type within which the reacting liquid or slurry contents are maintained automatically at a constant level. Into this reactor there are continuously introduced fresh methanol, carbon monoxide, sufficient water as needed to maintain at least a finite concentration of water in the reaction medium, recycled catalyst solution from the flasher base, a recycled methyl iodide and methyl acetate phase, and a recycled aqueous acetic acid phase from an overhead receiver decanter of the methyl iodide acetic acid light ends or splitter column 14. Distillation systems are employed that provide means for recovering the crude acetic acid and recycling catalyst solution, methyl iodide, and methyl acetate to the reactor. In a preferred process, carbon monoxide is continuously introduced into the carbonylation reactor just below the agitator, which is used to stir the contents. The gaseous feed is thoroughly dispersed through the reacting liquid by this stirring means. A gaseous purge stream is vented from the reactor to prevent buildup of gaseous by-products and to maintain a set carbon monoxide partial pressure at a given total reactor pressure. The temperature of the reactor is controlled and the carbon monoxide feed is introduced at a rate sufficient to maintain the desired total reactor pressure.

Liquid product is drawn off from the carbonylation reactor at a rate sufficient to maintain a constant level therein and is introduced to the flasher. In the flasher the catalyst solution is withdrawn as a base stream (predominantly acetic acid containing the rhodium and the iodide salt along with lesser quantities of methyl acetate, methyl iodide, and water), while the vapor overhead stream of the flasher comprises largely the product acetic acid along with methyl iodide, methyl acetate, and water. Dissolved gases exiting the reactor and entering the flasher consist of a portion of the carbon monoxide along with gaseous by products such as methane, hydrogen, and carbon dioxide and exit the flasher as part of the overhead stream. The overhead stream is directed to the light ends or splitter column 14 as stream 26.

It has been disclosed in U.S. Pat. Nos. 6,143,930 and 6,339,171 that there is a higher concentration, about 3 times, of the PRC's and in particular acetaldehyde content in the light phase than in the heavy phase stream exiting column 14. Thus, in accordance with the present invention, stream 28, containing PRC's is directed to an overhead receiver decanter 16 where the light ends phase, stream 30, is directed to distillation column 18.

The present invention may broadly be considered as a method of removing PRC's, primarily aldehydes and alkyl iodides, from a vapor phase acetic acid stream. The vapor phase stream is distilled and extracted to remove PRC's. An exemplary method of removing aldehydes and alkyl iodides and reducing levels of propionic acid, from a first vapor phase acetic acid stream includes the following steps: a) condensing the first vapor phase acetic acid stream in a first condenser and biphasically separating it to form a first heavy liquid phase product and a first light liquid phase product; b) distilling the light liquid phase product in a first distillation column to form a second vapor phase acetic acid product stream which is enriched with aldehydes and alkyl iodides with respect to the first vapor phase acetic acid stream; c) condensing the second vapor phase stream in a second condenser to form a second liquid phase product; d) distilling the second liquid phase product in a second distillation column to reduce and/or remove the alkyl iodide, aldehyde, and propionic acid impurities in the first vapor phase acetic acid stream into an overhead aldehyde and alkyl iodide stream; and e) measuring the density of the overhead stream, optionally calculating therefrom the relative concentrations of acetaldehyde and methyl iodide, and controlling the operation of the second distillation column based on the measured density or the concentrations calculated therefrom.

An embodiment of the prior art as disclosed in U.S. Pat. No. 6,339,171 is shown in FIG. 1. Referring to FIG. 1, the first vapor phase acetic acid stream (28) contains methyl iodide, methyl acetate, acetaldehyde and other carbonyl components. This stream is then condensed and separated (in vessel 16) to separate the heavy phase product containing the larger proportion of catalytic components—which is recirculated to the reactor (not shown in FIG. 1), and a light phase (30) containing acetaldehyde, water, and acetic acid.

Either phase of the light ends overhead may be subsequently distilled to remove the PRC's and primarily the acetaldehyde component of the stream, although it is preferred to remove PRC's from the light phase (30) because it has been found that the concentration of acetaldehyde is somewhat greater in that phase. In the embodiment depicted and described herein, the distillation is carried out in two stages; but it will be appreciated that the distillation may be performed in a single column as well. The light phase (30) is directed to column 18, which serves to form a second vapor phase (36) enriched in aldehydes and alkyl iodides with respect to stream 28. Steam 36 is condensed (vessel 20) to form a second liquid phase product. The second liquid phase (40) containing acetaldehyde, methyl iodide, methanol, and methyl acetate is directed to a second distillation column (22) wherein the acetaldehyde is separated from the other components. This process has been found to reduce and/or remove at least 50% of the alkyl iodide impurities found in an acetic acid stream. It has also been shown that acetaldehyde and its derivatives are reduced and/or removed by at least 50%, most often greater than 60%. As a result, it is possible to keep the concentration of propionic acid in the acetic acid product below about 400 parts per million by weight, preferably below about 250 parts per million.

From the top of the light ends or splitter column, 14, vapors are removed via stream 28, condensed, and directed to vessel 16. The vapors are chilled to a temperature sufficient to condense and separate the condensable methyl iodide, methyl acetate, acetaldehyde and other carbonyl components, and water into two phases. A portion of stream 28 comprises noncondensable gases such as carbon dioxide, hydrogen, and the like and can be vented as shown in stream 29 on FIG. 1. Also leaving overhead receiver decanter 16, but not illustrated in FIG. 1, is the heavy phase of stream 28. Ordinarily this heavy phase is recirculated to the reactor, but a slip stream, generally a small amount, e.g., 25 vol. %, preferably less than about 20 vol. %, of the heavy phase may also be directed to a carbonyl treatment process and the remainder recycled to the reactor or reaction system. This slip stream of the heavy phase may be treated individually or combined with the light phase (stream 30) for further distillation and extraction of carbonyl impurities.

The light phase (stream 30) is directed to distillation column 18. A portion of stream 30 is directed back to the light ends column 14 as reflux stream 34. The remainder of stream 30 enters column 18 as stream 32 in about the middle of the column. Column 18 serves to concentrate the aldehyde components of stream 32 into overhead stream 36 by separating water and acetic acid from the lighter components. Stream 32 is distilled in first distillation column 18, which preferably contains approximately 40 trays, and temperature ranges therein from about 283° F. (139.4° C.) at the bottom to about 191° F. (88.3° C.) at the top of the column. Exiting the bottom of 18 is stream 38 containing approximately 70% water and 30% acetic acid. Stream 38 is processed, generally cooled utilizing a heat exchanger, is recycled to the light ends column overhead decanter 16 via streams 46, 48 and ultimately to the reactor or reaction system. It has been found that recycling a portion of stream 38 identified as stream 46 back through decanter 16 increases efficiency of the inventive process and allows for more acetaldehyde to be present in the light phase, stream 32. Stream 36 has been found to have approximately seven times more aldehyde content when stream 38 is recycled through decanter 16 in this manner. Exiting the top of column 18 is stream 36 containing PRC's and in particular acetaldehyde, methyl iodide, methyl acetate, and methanol, and alkyl iodides. Stream 36 is then directed to an overhead receiver 20 after it has been chilled to condense any condensable gases present.

Exiting overhead receiver 20 is stream 40 containing acetaldehyde, methyl iodide, methyl acetate, and methanol. A portion of stream 40 is returned to column 18 as reflux stream 42. The remainder of stream 40 enters second distillation column 22 close to the bottom of the column. Column 22 serves to separate the majority of the acetaldehyde from the methyl iodide, methyl acetate, and methanol in stream 40. In one embodiment, column 22 contains about 100 trays and is operated at a temperature ranging from about 224° F. (106.6° C.) at the bottom to about 175° F. (79.4° C.) at the top. In an alternate, preferred embodiment, column 22 contains structured packing in place of trays. The preferred packing is a structured packing with an interfacial area of about 65 ft²/ft³, preferably made from a metallic alloy like 2205 or other like packing material, provided it is compatible with the compositions to be purified in the column. It was observed during experimentation that uniform column loading, which is required for good separation, was better with structured packing than with trays. Alternatively, ceramic packing may be employed. The residue of column 22, stream 44, exits at the bottom of the column and is recycled to the carbonylation process. It will be apparent to persons of ordinary skill that the separations performed in distillation columns 18 and 22 could also be carried out using a single distillation column.

Acetaldehyde polymerizes in the presence of methyl iodide to form metaldehyde and paraldehyde. These polymers generally are low molecular weight, less than about 200. Paraldehyde has been found to be relatively soluble in the reaction liquid, and primarily in acetic acid. Metaldehyde, upon its precipitation, is a sand-like, granular polymer that is not soluble in the reaction liquid beyond about 3 wt % concentration.

As disclosed in U.S. Pat. No. 6,339,171, however, it has been discovered that during the reaction, and with the heating of column 22, higher molecular weight polymers of acetaldehyde form. These higher molecular weight polymers (molecular weight greater than about 1000) are believed to form during processing of the light phase and are viscous and thixotropic. As heat is applied to the system, they tend to harden and adhere to the walls of the tower where their removal is cumbersome. Once polymerized, they are only slightly soluble in organic or aqueous solvents and can be removed from the system only by mechanical means. Thus an inhibitor is needed, preferably in column 22, to reduce the formation of these impurities, i.e., metaldehyde and paraldehyde and higher molecular weight polymers of acetaldehyde (AcH). Inhibitors generally consist of C₁₋₁₀ alkanols, preferably methanol; water; acetic acid and the like used individually or in combination with each other or with one or more other inhibitors. Stream 46, which is a portion of column 18 residue and a slip stream of stream 38, contains water and acetic acid and hence can serve as an inhibitor. As shown in FIG. 1, stream 46 splits to form streams 48 and 50. Stream 50 is added to column 22 to inhibit formation of metaldehyde and paraldehyde impurities and higher molecular weight polymers. Since the residue of second column 22 is recycled to the reactor, any inhibitors added must be compatible with the reaction chemistry. It has been found that small amounts of water, methanol, acetic acid, or a combination thereof, do not interfere with the reaction chemistry and practically eliminate the formation of polymers of acetaldehyde. Stream 50 is also preferably employed as an inhibitor since this material does not change the reactor water balance. Although water is not particularly preferred as an inhibitor, other important advantages are obtained by adding water to column 22.

Exiting the top of column 22 is stream 52 containing PRC's. Stream 52 is directed to a condenser and then to overhead receiver 24. After condensation, any non-condensable materials are vented from receiver 24; the condensed materials exit receiver 24 as stream 54. Stream 56, a slip stream of stream 54, is used as reflux for column 22. Exiting the bottom of column 22 is stream 44 containing methyl iodide, methanol, methyl acetate, methanol and water. This stream is combined with stream 66, which will be described below, and directed to the reactor.

It is important for the extraction mechanism that the overhead stream of column 22 remain cold, generally at a temperature of about 13° C. This stream may be obtained or maintained at about 13° C. by conventional techniques known to those of skill in the art, or any mechanism generally accepted by the industry.

Upon exiting receiver 24, stream 58 is preferably sent through a condenser/chiller (now stream 62) and then to an extractor 27 to remove and recycle small amounts of methyl iodide from the aqueous PRC stream. In extractor 27, PRC's and alkyl iodides are extracted with water, preferably water from an internal stream so as to maintain water balance within the reaction system. As a result of this extraction, methyl iodide separates from the aqueous PRC's and alkyl iodide phase. In a preferred embodiment, a mixer-settler with a water-to-feed ratio of about 2 is employed.

The aqueous extract stream 64 leaves the extractor from the top thereof. This PRC-rich, and in particular, acetaldehyde-rich aqueous phase is directed to waste treatment. Also exiting the extractor is raffinate stream 66 containing methyl iodide, which is normally recycled to the reaction system and ultimately to the reactor.

The present applicants have now discovered that it is advantageous to analyze the composition of the condensed overhead 54 from second column 22 and to use that analysis to provide feedback control of the distillation process. While it is extremely desirable to remove as much acetaldehyde and other PRC's as possible from the acetic acid process, it is important to do so without sacrificing cost-effectiveness. A major aspect of the process described herein is that because methyl iodide is an especially expensive material that is also very expensive to dispose of as a result of safe handling issues, it is especially desirable to implement process improvements that can remove acetaldehyde to prevent formation of alkyl iodides and PRC's but at the same time conserve methyl iodide to the greatest extent possible. It will be appreciated that the challenge of meeting these objectives at the same time is not insubstantial because methyl iodide and acetaldehyde have similar boiling points, making it somewhat difficult to achieve an optimal separation. As one of ordinary skill in the art would appreciate, the distillation process to separate methyl iodide from acetaldehyde is very sensitive to relatively minor fluctuations in temperature, reflux ratio and the like. Consequently it is desirable to have the most accurate process information available concerning the quality of the methyl iodide/acetaldehyde separation.

The applicants have found that the distillation process described above can be controlled with greater accuracy by measuring the relative concentrations of methyl iodide and acetaldehyde in the condensed distillate in stream 54, 56, 58 or 62. Surprisingly, this can be accomplished simply by measuring the density of the distillate. Unlike acetaldehyde, which has a density of about 0.78 g/cm³ at room temperature (similar to many common organic compounds), methyl iodide has a density of about 2.3 g/cm³, almost three times greater. This density difference is sufficiently great that the relative concentrations of methyl iodide and acetaldehyde in a mixture of the two compounds may be calculated directly from the density. The density may be measured either at normal process conditions or after cooling a sample to room temperature. It is preferred to measure the density under actual process conditions to eliminate unnecessary time lag in the control loop that would be introduced by a pre-cooling period.

The density may be measured in any of streams 54, 56, 58, or 62 (all of which have the same composition) using a conventional online densitometer denoted as 70 in FIGS. 2-4. For example, the applicants conducted a series of experiments to correlate the measured density of the overhead stream 54 with measured concentrations of methyl iodide (MeI), acetaldehyde (AcH), and dimethyl ether (DME). The following data were obtained:

TABLE 1 Correlation of AcH and Mel Concentration with Density Sample No. 1 2 3 4 5 6 DME (wt %) 8.3 6.6 3.4 4.6 3.6 3.8 AcH (wt %) 59.2 49.8 27.9 35.3 32.5 2.2 Mel (wt %) 32.5 43.6 68.8 60.1 63.9 93.8 Density 0.9793 1.0524 1.4082 1.3031 1.3166 1.6337 online (g/mL) Density 0.9925 1.0832 1.4464 1.3334 1.3577 1.688 @ 20° C. (g/mL)

FIG. 5 depicts the correlation of methyl iodide and acetaldehyde concentrations with the online density measurements. Reasonably linear trends were observed for both the methyl iodide and acetaldehyde concentrations, indicating that under typical process conditions both concentrations can be calculated from a single density measurement.

These density measurements, or calculated relative concentrations based thereon, may serve as the basis for controlling the distillation process in column 22 to optimize the separation of methyl iodide and acetaldehyde. This can be accomplished, for example, by increasing or decreasing the heat flow rate to the distillation column in response to changes in the ratio of methyl iodide to acetaldehyde. Alternatively, the reflux rate within the column may be adjusted (for example, by varying the split between streams 58 and 56) in response to the concentration ratio. As a further alternative, the concentration of the column reflux may be adjusted in response to the measured concentrations of acetaldehyde and methyl iodide by increasing or decreasing the flow rate of stream 50. This can be accomplished, for example, by adjusting the split of slip stream 46 between streams 48 and 50. Column pressure can also be controlled in response to the calculated concentrations.

The composition of the column feed may also be adjusted. It has been discovered that under certain circumstances it is advantageous to separate and recirculate a portion of stream 66 to the column feed 40 of column 22. Changing the flow rate of this stream in response to changes in the measured relative concentrations of methyl iodide and acetaldehyde would in turn alter the column feed composition.

Control schemes that allow modification of more than one of the process parameters associated with the distillation column in response to measured changes in the composition of the distillate are also within the scope of the present invention. For example, it may be appropriate to control both feed and reflux compositions at the same time, to adjust the column heat rate to account for changing feed compositions, or to adjust the reflux rate to offset changes in reflux composition. Many other similar variations are possible.

In addition to measuring the density of one or more of the streams mentioned above for the purpose of controlling the distillation column, it is also desirable to employ density measurements to monitor or control the operation of extractor 27. It will be understood that extractor 27 functions by facilitating a phase separation therein between a heavy methyl iodide phase and a less dense, aqueous phase containing acetaldehyde. Consequently, a significant change in the measured density of either the aqueous extract stream 64 or the methyl iodide-rich raffinate stream 66 would indicate a loss of phase separation in the extractor, which in turn would indicate that methyl iodide was being removed in the extract stream. As has been explained herein and elsewhere, it is desirable to conserve methyl iodide and reuse it within the process to the maximum extent practical; in addition, the presence of methyl iodide in the aqueous extract can adversely affect the wastewater treatment process to which the extract is ordinarily subjected. Similarly, measuring the density of stream 66 permits monitoring of the residual acetaldehyde concentration in that stream, allowing for corrective action (e.g. an increase in water flow to extractor 27) in response to an unacceptably high acetaldehyde concentration. In a further refinement of the invention, a single densitometer may be equipped to selectively measure the density of any one of multiple streams within the process.

While the invention has been described with reference to the preferred embodiments, obvious modifications and alterations are possible by those of ordinary skill in the related art. In particular, although the present invention has been generally described above utilizing the light ends phase of column 14, any stream in the carbonylation process having a high concentration of PRC's and alkyl iodides may be treated in accordance with the present invention. Similarly, although the process has been described above with respect to an acetaldehyde removal system of a particular configuration, minor variations of the disclosed configuration, for example replacing distillation columns 18 and 22 with a single column, are also contemplated. Therefore, the invention includes all such modifications and alterations to the full extent that they come within the scope of the following claims or the equivalents thereof. 

1. A process for producing acetic acid comprising the steps of: reacting methanol with carbon monoxide in a reaction medium comprising water, methyl iodide, and methyl acetate in the presence of a Group VIII metal carbonylation catalyst; separating the products of said reaction into a volatile product phase comprising acetic acid, and a less volatile phase; distilling said volatile phase in a distillation apparatus to yield a purified acetic acid product and a first overhead comprising methyl iodide and acetaldehyde; condensing at least a portion of said first overhead; measuring the density of said condensed first overhead; and adjusting at least one process control parameter associated with the distillation of said volatile phase in response to said measured density or a concentration calculated therefrom.
 2. The process of claim 1, wherein the purified acetic acid product contains less than about 400 parts per million by weight of propionic acid.
 3. The process of claim 2, wherein the purified acetic acid product contains less than about 250 parts per million by weight of propionic acid.
 4. The process of claim 1, wherein said at least one process control parameter is selected from the group consisting of a rate of heating said distillation apparatus, a composition of the feed or reflux to a column in said distillation apparatus, a reflux ratio in said distillation apparatus, a column pressure in said distillation apparatus, and combinations thereof.
 5. The process of claim 1, wherein said distilling step comprises at least two successive distillations, said process further comprising the step of supplying at least a portion of a residuum from one of said distillations as a reflux stream in another of said distillations, wherein the proportion of said residuum supplied as reflux is adjusted in response to said measured density or a concentration calculated therefrom.
 6. The process of claim 1, further comprising the step of extracting said condensed first overhead with water to produce a raffinate comprising methyl iodide.
 7. The process of claim 6, wherein a flow rate of water associated with said extraction step is adjusted in response to said measured density or a concentration calculated therefrom.
 8. The process of claim 4, further comprising mixing at least a portion of said raffinate with a feed or overhead stream associated with said distillation apparatus, wherein the fraction of said raffinate that is mixed with the feed or overhead stream is adjusted in response to said measured density or a concentration calculated therefrom.
 9. A process for producing acetic acid comprising the steps of: reacting methanol with carbon monoxide in a reaction medium comprising water, and methyl iodide in the presence of a Group VIII metal carbonylation catalyst; performing a liquid-vapor separation of said reaction medium to provide a vapor phase comprising acetic acid, methyl iodide, acetaldehyde and water and a liquid phase; distilling said vapor phase in a distillation apparatus to produce a purified acetic acid product and at least a first overhead comprising acetaldehyde and methyl iodide; condensing said first overhead; extracting said first overhead with water to produce a raffinate comprising methyl iodide and an aqueous extract; measuring the density of at least one stream selected from the group consisting of said first overhead, said raffinate and said aqueous extract; and adjusting at least one process control parameter associated with either the distillation of said vapor phase or the extraction of said first overhead in response to said measured density or a concentration calculated therefrom.
 10. The process of claim 9, wherein said at least one process control parameter is selected from the group consisting of a rate of heating said distillation apparatus, a composition of a feed or reflux stream associated with a column in said distillation apparatus, a reflux ratio in said distillation apparatus, a pressure in said distillation apparatus, a water feed rate to said extraction step, and combinations thereof.
 11. The process of claim 9, further comprising maintaining the concentration of propionic acid in said acetic acid product below about 400 parts per million by weight.
 12. The process of claim 11, further comprising maintaining the concentration of propionic acid in said acetic acid product below about 250 parts per million by weight. 